Purpose: We present on the first reported instance of x-ray-induced MR signal changes in a water phantom and investigate a potential mechanism. Radiation-produced free radicals cause significant indirect DNA damage during radiotherapy. Quantifying or imaging these radicals in vivo would potentially have substantial benefits for dosimetry and treatment verification. To realize this, it is necessary to characterize the physics between radical concentration and T1 change, a quantity known as relaxivity. The literature reports that hydroxyl radical concentrations of 0.01nM can shorten T1 by 35%. Here, we test the hypothesis that MR-linac x-rays produce a steady-state hydroxyl radical concentration in distilled water sufficient to induce a measurable T1 change.
Methods: Homogeneous-stage radiochemistry simulations were performed to calculate radical concentrations in distilled water (DW) versus 10-mM coumarin solution (CW), which scavenges hydroxyl radicals. Time-averaged radical concentrations were converted to T1 shortening using a reported hydroxyl radical relaxivity value. Images were acquired on a 0.35-T MR-linac during beam delivery using a spin-echo water-nullified inversion-recovery (IR) sequence sensitive to millisecond-scale T1 changes.
Results: DW simulation estimated a time-averaged hydroxyl radical concentration of 0.03nM during beam-on, which should be sufficient to shorten T1 by 1100ms. Imaging results were inconsistent with this finding. In a DW phantom, preliminary IR imaging showed no signal change in the irradiated region. In contrast, a CW phantom showed a small signal change within the irradiated region. Simulation of a CW solution estimated a time-averaged hydroxyl radical concentration of 0.1pM, which should be sufficient to shorten T1 by 10ms. In addition, this signal change persisted for minutes after beam-off, suggestive of an alternative mechanism.
Conclusion: MR signal changes induced by x-ray irradiation in a water phantom are observed and presented for the first time. These signal changes may not be due to paramagnetic relaxation enhancement by hydroxyl radicals.